I would use black 3M strip chalking. Pretty much what everyone is using
Here are a couple of original examples from around that time and from that plant. Its not unusual for this stuff to get rock hard and fall off over the years.
Ran across this one as I was looking for yours - thought it was interesting. What did they do when the owner didn't order the windshield washers? They still plugged the holes or at least they did at this plant around that same time